The X-51A Hypersonic Plane: What Went Wrong?

Last summer's flight of the X-51A hypersonic aircraft ended in failure even before engineers on the ground could light the engine. The head of the program explains why, and gives us a hint of what's next.

The payoff for reaching air-breathing hypersonic flight—five times the speed of sound, without rockets—would be revolutionary. A hypersonic airliner could carry passengers to any place on Earth within four hours using conventional airports and infrastructure. And then there are the incredible military applications of such speed: imagine a cruise missile that could strike a target 500 miles away in just 10 minutes.

The Air Force and DARPA have been partnering on the X-51A unmanned scramjet project, the most advanced hypersonic aircraft yet built. But multiple tests have so far ended in abrupt failure. Most recently, the plane lost control during a test in August before it even could start its scramjet engine. The Air Force—which administers the program at the Air Force Research Laboratory in Ohio— initially offered few details about what went wrong. An ongoing investigation has by now narrowed the cause to a premature unlatching and subsequent failure of a control fin, according to program manager Charlie Brink.

As planned, the X-51A dropped from a B-52 bomber out of Edwards Air Force Base in California. Then a rocket attached to the back boosted the vehicle to its operating speed. But, Brink told reporters on a conference call this week, during the boost, one of the aircraft's four control fins became unlatched, apparently disabling the actuator needed to control the X-51A in free flight after the booster drops off.

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After the rocket booster and its controlling influence dropped away, said Brink, "We had three fins working really hard to control a vehicle that needs four fins." The vehicle went out of control only a second and a half later. "We did not even attempt—we didn't get to the point in the sequence where we start putting fuel into the engine and try and light the engine. So for this flight, we just didn't have a propulsion experiment," he said.

Although this is not yet confirmed, it appears that vibrations during the rocket boost shook the control fin free of its spring-loaded latches, which should make a fix relatively straightforward, Brink said. The fix could be as simple as unlimbering the control fins and powering up their actuators right after the X-51A drops from the bomber instead of waiting until they are actually needed when the booster falls away.

The unmanned X-51A is 25 feet long and weights about 4000 pounds. Its core technology is a supersonic combustion ramjet, or scramjet. Scramjets differ from ordinary jets in that they do away with compressors and turbines—the moving parts that limit the speed of the fastest jets to under Mach 3. Instead, once boosted to supersonic speed by means such as a rocket or conventional jet engine, a scramjet depends on the sheer force of incoming air to achieve the compression required for the high burn rate of hypersonic flight.

The hypersonic challenge has been likened to keeping a match lit in a hurricane because the extreme force of the air flowing into the engine makes sustained combustion difficult. It has plagued aerospace engineers for the last fifty years, with progress advancing painfully slowly. Test flights often occur years apart. It's a high-stress, high-risk undertaking for the researchers involved. When I first spoke with Brink before the first test flight, he told me he would be holed up in the men's room at Edwards Air Force Base throwing up when the test vehicles launched. "And I have been," he told me on the conference call.

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The goal of the X-51A program is to run a scramjet engine in flight until the fuel tanks run dry, proving out the possibility of controlled, indefinite duration air-breathing hypersonic flight. In terms of minutes flown, it's already the most successful hypersonic aircraft to leave the lab, even though it has yet to achieve its objective of 4.5 minutes of powered flight. The first of the four vehicles the program has constructed flew under its own power for 2.5 minutes in 2010; it broke the previous hypersonic flight record held by NASA's X-43, which stayed lit for all of 10 seconds in 2004. (That vehicle ran on hydrogen as fuel, while the X-51A runs on JP-7 jet fuel, which should make production vehicles easier to support because they will be able to use fueling infrastructure already in place for conventional aircraft.) The second X-51A shut down prematurely in a test in 2011 after only 10 seconds of powered flight.

Following this third test in August 2012, just one of the single-use X-51As remains. The $300 million program provides for a final flight, which should happen in spring or summer 2013. Even though the future of hypersonic research in the U.S. remains uncertain beyond that point, Brink expressed confidence that the Air Force will keep its hand in the game. "We're working with industry on follow-on programs," he said. He mentioned as a possible option the proposed Air Force High Speed Strike Weapon Demonstration Program, which would call for the successful demonstration of a hypersonic cruise missile. "But I really can't get into the details of that project right now," he said.

Michael Belfiore is the author of The Department of Mad Scientists: How DARPA Is Remaking Our World, from the Internet to Artificial Limbs and is a frequent PM contributor.

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